Such systems are characterized by the provision of an intermediate silo reserved for pulverized fuel between the solid fuel grinding station and the combustion chamber. This type of system is used for fuels that are difficult to burn, such as anthracite or fuels low in volatile material (e.g. low volatile bituminous coal as defined by the ASTM), since it clearly separates and optimizes the grinding of the solid fuel, on the one hand, and the combustion of said pulverized fuel, on the other hand.
There are a number of types of indirect heating system: those which return the exhaust (products obtained by separating the pulverized fuel and the gases used to dry and transport said fuel) to the combustion chamber and those which reject the exhaust to the atmosphere.
These systems operate in the following manner: the solid fuel, for example coal, is stored in a silo and conveyed to a grinder in which it is ground and dried by very hot air or gas. The pulverized fuel is then transported pneumatically to a separator, which intercepts the coarse particles and returns them to the inlet of the grinder, and thence into one or more cyclones which intercept the pulverized fuel and discharge it into an intermediate storage silo; ducts and possibly a gas recirculation fan complete the installation.
The grinding system receives hot air and gases for drying the untreated fuel. The hot gases and those produced by the evaporation of the moisture from the fuel are surplus and must therefore be extracted via dedicated ducting known as exhaust ducting. All these circuits operate at a low temperature (approximately 100° C.).
The cyclone does not intercept all of the fine solid fuel particles in the surplus gases, and it is therefore necessary to treat these particles. The usual concentrations of solid fuel in the gases are of the order of 50 g/m3 to 200 g/m3.
The ultrafine particles can either be sent to the combustion chamber by a fan (return of exhaust to the combustion chamber) or intercepted in a dust extractor such as an electrostatic dust extractor or a bag filter and discharged into the intermediate silo, where they are mixed with the other particles from the cyclone (rejection of exhaust to the atmosphere).
These techniques are satisfactory for most fuels, but they perform much less well for the fuels lowest in volatile material. In particular, it is necessary to introduce noble fuels such as fuel oil or natural gas to support combustion at low loads.
If the exhaust is returned to the combustion chamber, the gases transporting the ultrafine particles are essentially inert gases, such as water vapor caused by evaporation of the moisture from the fuel, and because they come from hot combustion gases taken from the combustion chamber to dry the untreated fuel these gases have high concentrations of CO2 and low concentrations of O2. Moreover, the concentration of fuel particles in the gases is far from the optimum required for good combustion.
Combustion of the particles is all the more difficult to achieve in that a compromise must be found between injecting particles sufficiently near the main flame for the ignition of the ultrafine particles to benefit from the high temperatures in this area and sufficiently far from the flame for the latter not to be disturbed by the mass of relatively cold inert gases injected with the ultrafine particles. Choosing the injection area is therefore particularly difficult and requires very considerable experience.
If the exhaust is rejected to the atmosphere, in the case of fuels which are difficult to burn because they are lower in volatile materials, the ultrafine particles from the dust extractor are mixed with the coarser fuel in the intermediate silo, and these improved particles therefore cannot be exploited.